The present invention relates to a method for improving performance of reading a file by using a plurality of tape media. More specifically, the present invention relates to a method for reading a file at a high speed from two media holding identical files by using a file system.
LTFS (Linear Tape File System) has been put into practical use as a mechanism allowing a tape drive (magnetic tape drive) to access data stored on tape media (media or tapes) as files. LTFS associates pieces of meta information, such as information about where data portions of files exist on tapes, with one another as Index (indices), thereby implementing a file system.
When data is held in a tape drive, high reliability and performance enabling high-speed file reading and writing are always required of LTFS. A long seek time taken to seek a data position where a file is stored is a drawback which is unavoidable in the case where files are stored in a tape drive.
This performance drawback is caused by the following reasons:
A tape drive is a sequential access storage device, and running the tape from one end thereof to the other in the longitudinal direction takes approximately two minutes. A medium on which data is to be written is divided into wraps at positions in the widthwise direction of data writing. A long time is taken to change the writing direction at the end of each wrap (wrap change). While data is being successfully read, for example, fifth-generation LTO (Linear Tape Open) (hereinafter, referred to as “LTO5”) tape drives have a read speed of 140 MB/sec, which is comparable to the level of HDDs. However, once LTO5 tape drives fail to read data, they re-read the data from the same position to perform an error recovery procedure (ERP). This re-reading requires a backhitch operation for rewinding the tape, and becomes a cause of the performance degradation.
The primary purpose for the user to store data on tape media is to store data at high reliability. For this purpose, instead of storing data only on one highly reliable medium, the same data is generally stored additionally on another medium for backup. Accordingly, it is necessary to consider achieving a higher speed by efficiently reading data written on two media, namely, main and backup media.
Japanese Patent Application Publication No. 2005-148854 discloses high-speed data reading by using a technique including a plurality of disk devices. Reading a file at a high speed by using a plurality of tape media is also strongly required of a storage system including a plurality of tape drives.